Assessment of microbial activity in water based on hydrogen peroxide decomposition rates

This study proposes a new and simple assay that allows rapid assessment of microbial activity in water samples. The assay consists of standardized hydrogen peroxide (H2O2) addition to a water sample and subsequent spectrophotometric determination of H2O2 reduction over time. The H2O2 decomposition rate constant reflects the level of enzymatic activity from planktonic and particle-associated bacteria as well as algae and protozoans. The proof of concept was verified on water samples from recirculating aquaculture systems (RAS), showing that the vast majority of H2O2 decomposition was related to microbial activity. Only 3% of the total H2O2 decomposition was related to abiotic processes when 0.20 mu m sterile filtered RAS water was compared with unfiltered RAS water. Planktonic bacteria (size range 0.20-1.6 mu m) accounted for 16% of H2O2 decomposition, while bacterial aggregates, particle-associated bacteria and microbiota above 1.6 mu m were responsible for the remaining 81%. H2O2 decomposition rate constants were positively correlated to BOD5 (r = 0.893; p < 0.001; n = 18) and to the number of 1-30 mu m micro particles (r = 0.909; p < 0.001; n = 72) in RAS water, substantiating the biologically mediated decomposition processes in the water phase. The H2O2 decomposition assay thus represents a new alternative to existing methods that allows rapid (1-2 h) and simple quantification of microbial activity in fresh- and saltwater samples from aquaculture systems. Potential applications of the assay are discussed.